[Jaxonalden]

If you have so much crap in your oil that the filter plugs-up and can't flow through the media the oil pressure will override the spring tension (differential pressure, PSID). This will allow the oil to by-pass the filter media and continue to provide the engine with oil for lubrication.

It will be dirty oil, but if your filter is so contaminated causing a by-pass you have more problems than lubricating with dirty oil.

P.S. To better explain; in a perfect world with oil flowing through a filter and no restriction, the pressure is equal on both sides of the filter media (paper). When the filter becomes clogged these's a pressure differential and that differential pressure is what will cause the spring to compress and open, allowing oil to by-pass. Now the argument about by-passing during high RPM's, if the flow is so great the media won't flow the oil OR during cold start-up when the oil is thicker I can see it.